1. Field of the Invention
The present invention relates to graphite materials used for negative electrodes of lithium-ion secondary batteries and a method for manufacturing the same. More specifically, the present invention relates to graphite materials used for negative electrodes of lithium-ion secondary batteries with suppressed capacity degradation and high durability, negative electrodes using the same and lithium-ion secondary batteries including the negative electrodes.
2. Description of Related Art
Lithium-ion secondary batteries are light in weight and have high input and output characteristics compared with conventional secondary batteries such as a nickel-cadmium battery, a nickel-metal hydride battery and a lead battery, and such lithium ion secondary batteries have been anticipated in recent years as power supplies for electric vehicles and hybrid vehicles. Typically, these kinds of batteries include a lithium-containing positive electrode enabling reversible intercalation of lithium and a negative electrode including a carbon material, and these electrodes are opposed to each other via a non-aqueous electrolyte. As such, these kinds of batteries are assembled in a discharged state, and so will not be in a dischargeable state without charging. The following describes the charge and discharge reaction by way of an example including a lithium cobalt oxide (LiCoO2) as the positive electrode, a carbon material as the negative electrode and a non-aqueous electrolyte solution containing lithium salt as an electrolyte.
During charge of a first cycle, lithium contained in the positive electrode is firstly released to the electrolyte solution (the following Formula 1), so that the positive electrode potential shifts to a noble (positive) direction. At the negative electrode, lithium released from the positive electrode is occluded by the carbon material (the following Formula 2), so that the negative electrode potential shifts to a less noble direction. Typically when a difference between the positive electrode potential and the negative electrode potential, i.e., battery voltage reaches a predetermined value, the charge is terminated. This value is called a charge termination voltage. Then, during discharging, lithium occluded by the negative electrode is released, so that the negative electrode potential shifts to a noble direction, and the lithium is occluded again by the positive electrode, so that the positive electrode potential shifts to a less noble direction. Similarly to the charging, discharge also is terminated when a difference between the positive electrode potential and the negative electrode potential, i.e., battery voltage reaches a predetermined value. That value is called a discharge termination voltage. The whole reaction formula of such charge and discharge will be as in the following Formula 3. In the following second cycle or later, the charge and discharge reaction (cycles) progresses as lithium moves between the positive electrode and the negative electrode.

In general, carbon materials used for negative electrode materials of lithium-ion secondary batteries are broadly divided into graphite materials and amorphous materials. Graphite carbon materials have an advantage of having higher energy density per unit volume than amorphous carbon materials. As such, graphite carbon materials are typically used as a negative electrode material in a lithium-ion secondary battery for mobile phones and laptop computers that are required to be compact in size but have large charge and discharge capacity. Graphite has a structure including regularly laminated reticulated planes of carbon atoms, and during charge and discharge an intercalation and deintercalation reaction of lithium ions progresses at the edges of the reticulated planes.
As stated above, these kinds of batteries has been examined actively for the use as electric storage devices for vehicles, industry, and electric power supply infrastructure in recent years. When these batteries are used for these purposes, they are required to have extremely high-degree of reliability compared with the usage for mobile phones or laptop computers. The term “reliability” is a property related to product life, referring to a property of hardly changing (hardly deteriorating) in charge and discharge capacity and the internal resistance during repeated charge and discharge cycles, during storage while being charged to be a predetermined voltage or while being charged (floating charged) continuously at a constant voltage.
Meanwhile, it is generally known that lithium-ion secondary batteries conventionally used for mobile phones and laptops have service life characteristics greatly depending on the negative electrode materials as well. One of the reasons is that, due to low charge and discharge efficiency at the negative electrode, it is impossible in principle to make the charge and discharge efficiency identical between the positive electrode reaction (Formula 1) and the negative electrode reaction (Formula 2). The charge and discharge efficiency refers to a ratio of dischargeable electric capacity to the electric capacity consumed by charging. The following is a detailed description on a reaction mechanism to degrade the service life characteristics due to such low charge and discharge efficiency of the negative electrode reaction.
During charging, lithium is released from the positive electrode (Formula 1) and is occluded by the negative electrode (Formula 2) as stated above, where the reactions at the positive electrode and the negative electrode consume the same amount of electric capacity during charging. The charge and discharge efficiency, however, is lower at the negative electrode, so that during the subsequent discharging reaction, the discharge is terminated in the state in which the amount of lithium released from the negative electrode is less than the amount of lithium that can be occluded on the positive electrode side, i.e., the amount of lithium that has been occluded before the charge on the positive electrode side. This is because a part of the electric capacity consumed by charge at the negative electrode is consumed by a side reaction and a competitive reaction and not by the reaction of occluding lithium, i.e., the occlusion reaction as dischargeable capacity.
As a result of such a charge and discharge reaction, the positive electrode potential at the discharge termination state shifts to a nobler direction than the original potential before the charge and discharge, and the negative electrode potential also shifts to a nobler direction than the original potential before the charge and discharge. This results from the following reason. All lithium, which has been released during the charge of the positive electrode, cannot be occluded (not return to) during discharging, and thus, during discharging to make the potential shifted to a noble direction during charging shift to a less noble direction, the potential cannot return to the original positive electrode potential by an amount corresponding to a difference in charge and discharge efficiency between the positive and negative electrodes. Then discharge is terminated at a nobler potential than the original positive electrode potential. As stated above, since the discharge of a lithium-ion secondary battery ends when the battery voltage (i.e., a difference between the positive electrode potential and the negative electrode potential) reaches a predetermined value (discharge termination voltage), a nobler potential at the positive electrode at the time of discharge termination means that the negative electrode potential shifts accordingly to a noble direction.
As stated above, during the repeated charge and discharge cycles, these kinds of batteries produce a problem of a reduction in capacity obtainable within a predetermined voltage range (within the range between the discharge termination voltage and the charge termination voltage) due to a change of an operation range of the capacity of the positive and negative electrodes. Such reaction mechanism of the capacity degradation has been reported in an academic conference, for example, as well (for example, Proceedings of the 48th Battery Symposium in Japan, 1A11 (Nov. 13, 2007) and Proceedings of the 76th Meeting of the Electrochemical Society of Japan, 1P29 (Mar. 26, 2009)). Once the operation range of the positive and negative electrode potentials changes, such a change is irreversible, and so the operation range cannot return to the original one in principle. There is no means to recover the capacity, which makes this problem more serious.
The above-described reaction mechanism causing capacity degradation during repeated charge and discharge cycles basically applies to a reaction mechanism for the capacity degradation during storage in a charged state or reaction mechanism for the capacity degradation during float-charge. When a battery is stored in a charged state, it is known that the amount of capacity lost by a side reaction and a competitive reaction which occur in a charged state is greater in the negative electrode than in the positive electrode, so that an operation range of the capacity of the positive and negative electrode changes between before and after storage and so the battery capacity after storage decreases (for example, Proceedings of the 71st Meeting of the Electrochemical Society of Japan, 2I07 (Mar. 24, 2004)). A difference in self discharge rate between the positive and negative electrodes under a charged state also owes to, similar to the above-mentioned difference in charge and discharge efficiency between the positive and negative electrodes, a side reaction or competitive reaction rate at the negative electrode under a charged state being higher than a side reaction or competitive reaction rate at the positive electrode in a charged state.
When a battery is float-charged, both the positive electrode and the negative electrode are charged at predetermined potentials continuously at the initial stage of charging. Actually, however, a current value (leakage current on the positive electrode side) necessary for keeping the positive electrode potential and a current value (leakage current on the negative electrode side) necessary for keeping the negative electrode potential are different. This results from, as described above, self discharge rates under a charged state are different between the positive electrode and the negative electrode and the self discharge rate of the negative electrode is larger. At the time of float charging, a leakage current becomes larger on the negative electrode side than on the positive electrode side, so that a negative electrode potential is shifted to the decreasing direction of a leakage current, that is, the noble direction and a positive electrode potential is shifted to the increasing direction of a leakage current, that is, the noble direction. In this way, during float-charge as well, an operation range of the capacity of the positive electrode and the negative electrode changes irreversibly, leading to degradation in battery capacity.
Lithium-ion batteries including a graphite negative electrode made up of highly developed crystals can yield high electric capacity. A battery including such a graphite material is said to have a tendency of, concurrently with insertion of lithium in graphite crystals, co-insertion and decomposition of the electrolyte solution from edge parts of the crystallites in between graphite layers each being a reticulated plane with high-degree of parallelism (solvent co-insertion model of Besenhard, J. O. Besenhard, M. Winter, J. Yang, W. Biberacher, J. Power Sources, 54, 228 (1995)). A side reaction and a competitive reaction resulting from the decomposition of the electrolyte solution between graphite layers cause a reduction in charge and discharge efficiency of the negative electrode, thus causing capacity degradation. The tendency of the solvent co-insertion is said to increase in more developed graphite crystals. Thus, in order to suppress decomposition of the electrolyte solution due to the solvent co-insertion, a method of introducing disorder of a crystal structure at the surface of particles has been reported. Japanese Patent No. 4171259 describes, following pulverizing and classifying of a raw coke composition, a mechanochemical treatment performed thereto, whereby the crystal structure at the surface of particles can be disordered. The document describes, since such disorder of the crystal structure still remains as unorganized carbon after graphitization as the final step, the initial charge and discharge efficiency of the negative electrode can be improved (paragraph [0024] of Japanese Patent No. 4171259). The disorder of the crystal structure introduced by a mechanochemical treatment, however, is a state in which unorganized carbon crystallites are oriented at random, that is, a so-called isotropic state, in which many edge parts are probably exposed to the surface of particles.
In general, there are a large number of dangling bonds at the edge parts of crystallites, that is, a valence electron bond is not saturated and many localized electrons are present without a binding partner. On the surface of a negative electrode carbon material during charge, that is, at the interface where an electrolyte solution comes into contact with the carbon material, a side reaction or a competitive reaction occurs because the localized electrons catalytically act to cause reduction decomposition of the electrolyte solution in addition to the intended charging reaction causing insertion of lithium in graphite crystals, thus decreasing charge and discharge efficiency of the negative electrode. That is, although unorganized carbon introduced at the surface of particles may suppress the decomposition of electrolyte solution due to the solvent co-insertion, the edges will be exposed to the surface because crystallites of the unorganized carbon introduced are in an isotropic state, resulting in the still-remaining problem of increasing reduction decomposition of the electrolyte solution and causing capacity degradation.